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TPS54218 Loop Compensation has wrong value for F_ESR_zero

Bill Strausser
Prodigy 645 points

All,

Please see attached schematic file and Bode plot file from the Switcher Pro desktop version. Basically when I use an Aluminum type Cout which has 120uF value and an ESR of 25mOhm, I should get a 53kHz ESR zero frequency indicated by Switcher Pro. Also this zero frequency should show up in the Power stage Bode plot.  

The result is I get is an F_esr_zero value displayed by SwitcherPro in the title area of  661kHz. This basically moves the zero frequency way away from where it should be and we do not display the correct ESR zero location in the Bode plot. . So Loop compensation is not close to being realistic. Can you review and let me know how to make this work correctly?

Thanks,

BIll 

Apparently I cannot attach more than one file so send me an E_mail and I will send files. w-strausser@ti.com

1V2_1A4_IBM_SwProErrSCH.ppt
  • 0
    TI__Guru**** 191445 points
    I already know why you are having a problem based on your schematic.  I see you have both output capacitor locations populated with different types of capacitors.  Switcherpro does not support that feature at this time.  It probably trying to calculate a single ESR zero based on the parallel combination of the two capacitors.  In reality, the parallel capacitors have a zero/pole/zero set of residuals.